Oceanside , New York -LRB- CNN -RRB- -- Deployed to a volatile outpost in southern Afghanistan where U.S. Marines routinely face a mix of skirmishes and hidden explosives , Greg Buckley Jr. sensed that an attack was imminent .

And he knew that it would come from within .

The 21-year-old Marine was posted to Garmsir in Helmand Province , where he was training local security forces as part of NATO 's planned withdrawal in 2014 .

It was during a static-filled phone call to his father over the summer that the Long Island native mentioned a run-in he had with an Afghan trainee while on guard duty .

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The encounter was the first in which the Buckley family 's eldest son seemed to sense something was wrong , according to what he told his family in phone conversations and a letter .

`` The guy turned around and said to Greg , ` We do n't want you here . We do n't need you here , ' '' his dad said .

`` Greg turned around again and said , ` Why would you say that ? ' '' according to Greg Buckley Sr. .

But the trainee apparently would n't relent , repeating the phrases for hours over the course of a night in which the young Marine was on guard watch .

`` Greg said , ' I thought I was going to lose my mind , ' '' his father said . `` Pitch black out , and all he kept saying over and over again is , ` We do n't want you . We do n't need you . We do n't want you . ' ''

`` It was just tormenting for him . ''

The two men then finally confronted each other , yelling until a group of officers separated them , he told his father .

`` One of his superiors came over and had Greg apologize to the guy , '' said the elder Buckley .

The 21-year-old agreed and extended his hand , but the man refused .

About a month later , Greg phoned his father again .

`` He told me if I have to stay here until November ... I 'm not going to come home . ''

Greg also asked his father to prepare to tell his mother and his two younger brothers that he 'd be killed .

`` I do n't understand , '' his father said . `` Out in the field ? '

`` No , in our base , '' Greg replied .

4 NATO troops killed in ` insider ' attack in Afghanistan

On August 10 , 2012 , Greg Buckley Jr. was gunned down by the very forces he had been training , just days after learning that he was to head home early .

`` It was only two days he had left there in Afghanistan , '' his father told CNN .

The phenomenon is known as `` green-on-blue , '' due to a color-coding system used by NATO . It has become disturbingly more frequent in Afghanistan , with more than 50 NATO troops killed this year by local forces , the first time that 's happened in a single year in the U.S.-led war .

Last year , 35 people died in such insider attacks , and even less the year before , according to NATO figures .

The killings have prompted suspensions of training new recruits while eroding the trust between NATO and its Afghan allies .

The gunman involved in Greg 's death attacked from inside his outpost and killed two other fellow Marines , his dad said .

But Greg Sr. said his son had informed his superior officers that `` one day they are going turn around and turn those weapons on us . ''

CNN can not independently confirm that Greg informed superior officers . Calls and emails to NATO 's International Security Assistance Force in Afghanistan were not immediately returned .

More than a decade after the war began , the Buckley family is now struggling to cope with the loss of its eldest son . More than 2,000 other U.S. service members have been killed in Operation Enduring Freedom .

`` It 's not really day-by-day , '' said Greg 's mother , Marina Buckley . `` It 's more minute-by-minute . ''

Back in Oceanside , the Buckley family on Friday attended the town 's first home football game , where their fallen son had intended to watch his youngest brother play varsity for the first time .

`` Greg was supposed to be home for this game , '' said Justin , 17 , who wore the number 30 on his back , Greg 's old basketball number .

`` I would tell him I love him and I miss him . ''

The senior running back , who donned a camouflage jersey along with his team , broke to the outside on Friday for a 25-yard score that helped cement the Sailors ' improbable second-half comeback against top-ranked East Meadow .

After crossing into the end zone , Justin raised his hand to salute -- honoring his fallen brother .

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Greg Buckley Jr. was gunned down by the forces he had been training

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`` He told me , if I have to stay here until November ... I 'm not going to come home , '' his dad said

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The phenomenon is known as `` green-on-blue '' and is disturbingly more frequent in Afghanistan

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Buckley warned superiors an insider attack could happen , his father says ; CNN could not confirm